This invention relates generally to giant magnetoresistive (GMR) sensors. More particularly, it relates to current-perpendicular-to-plane (CPP) magnetoresistive sensors.
Conventional magnetoresistive (MR) sensors, such as those used in magnetic recording disk drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensing electrical current or voltage.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. The physical origin of the GMR effect is that the application of an external magnetic field causes a variation in the relative orientation of magnetization of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a sandwich structure, called a spin valve, including two uncoupled or weakly coupled ferromagnetic layers separated by a nonmagnetic metal layer in which the magnetization of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the layer onto an antiferromagnetic layer which exchange-couples to the pinned layer. The unpinned layer or free ferromagnetic layer is free to rotate in the presence of small external magnetic field.
In GMR read heads, spin valves are typically arranged to operate in a CIP (current-in-plane) mode. In order to increase the recording density of a system, the track width of a read sensor must be made smaller. This reduces the signal available from a CIP sensor. However, in the CPP (current-perpendicular-to-plane) mode, the signal increases as the sensor width is reduced.
CPP spin valves have been oriented so that the bias current that is applied to the GMR films is perpendicular to the plane of the films. FIG. 1 shows a cross-sectional schematic diagram of a CPP spin valve 100 of the prior art. As shown in FIG. 1, CPP spin valve 100 includes a ferromagnetic free layer 106, a ferromagnetic pinned layer 102, a spacer layer 104 disposed between the ferromagnetic free layer 106 and the ferromagnetic pinned layer 102, and an anti-ferromagnetic layer 101 proximate the ferromagnetic pinned layer 102. A sensing electrical current 112 is perpendicular to the plane of the CPP spin valve 100""s layers (e.g., current flows vertically within the stack). The directions for the magnetizations of the ferromagnetic free layer 106 and the ferromagnetic pinned layer 102 are indicated. The spacer layer 104 is typically made of a conductive material such as a metal or metallic alloy.
One major problem of CPP spin valves is their low resistance. The resistance of a CPP spin valve is calculated by the following formula:
R=xcfx81(L/A)xe2x80x83xe2x80x83(1) 
Where
xcfx81 is the resistivity of the CPP spin valve;
L is the length of the CPP spin valve; and
A is the cross-sectional area of the CPP spin valve.
For example, a 100 Gbit/in2-CPP spin valve having a resistivity of about 10 xcexcxcexa9-cm (e.g., metallic structure), a length L of about 500 xc3x85, and a square cross section of about 500 xc3x85 on a side will have a resistance R of about 2 xcexa9. If 1 mA (i.e., 4xc3x9710xe2x88x927 amp/cm2) flows through this CPP spin valve which will typically exhibit a magnetoresistance (xcex94R/R) value of about 25%, then a voltage signal xcex94V obtained is about 0.5 mV peak-to-peak. Such a small resistivity given the low value of R thus translates into a small xcex94V which is difficult to detect.
Based on the formula (1), to increase the resistance of the CPP spin valve for achieving high voltage signal levels, one could either reduce the cross-sectional area A of the sensor, or increase the length L of the stack, or increases the resistivity xcfx81. One could manipulate the dimensions of the sensor, increasing L and decreasing A, to increase the sensor resistance. However, these dimensions are usually dictated by the recording density and cannot be altered much. Another approach involves increasing the resistivity of the CPP spin valve. However this usually involves introducing electron scattering sites to the sensor material, which tend to cause spin-flip scattering as well, and reduce AR/R.
There is a need, therefore, for an improved CPP magnetoresitive sensor, such as spin valve, that increases the resistance of the sensor and thus, the voltage signal xcex94V peak-to-peak, without reducing xcex94R/R.
Accordingly, it is a primary object of the present invention to provide a CPP magnetoresistive sensor with high resistance.
It is a further object of the present invention to provide a CPP magnetoresistive sensor with enhanced magnetoresistance (xcex94R/R).
It is an additional object of the present invention to provide a CPP magnetoresistive sensor generating high voltage signal levels.
It is a further object of the present invention to provide methods of making such a CPP magnetoresistive sensor.
These objects and advantages are attained by CPP magnetoresistive sensors with heterogeneous spacer layers.
According to a first embodiment of the present invention, a simple CPP spin valve includes a ferromagnetic free layer, a ferromagnetic pinned layer, a spacer layer disposed between the ferromagnetic free layer and the ferromagnetic pinned layer such that a magnetization of the ferromagnetic free layer is oriented by a magnetization of the ferromagnetic pinned layer. The CPP simple spin valve further includes an AF layer for pinning a magnetization of the ferromagnetic pinned layer. A sensing electrical current flows perpendicular with the CPP spin valve""s layers. The spacer layer includes a heterogeneous material, which is composed of conductive grains within a highly resistive matrix. The conductive grains are typically made of a conductive material, such as Cu, Au, Ag and their alloys, which operate as a GMR spacer material. The highly resistive matrix typically includes an insulating or highly resistive material, such as Si, SiO2, Al2O3, or NiO, which hinders the flow of electrons. The sensing electrical current travels from the ferromagnetic free layer to the ferromagnetic pinned layer through the conductive grains. Since no additional electron scattering sites are present in the conductive grains in the heterogeneous material of this type the magnetoresistance (xcex94R/R) is maintained. The heterogeneous materials of the spacer layer have high resistivities, which increase the resistance of the spacer layer, thus of the entire CPP spin valve. As a result, higher voltage signal levels can be obtained from the CPP spin valve.
Heterogeneous spacer layers can be used in CPP dual spin valves according to an alternative embodiment of the present invention. A CPP dual spin valve includes a ferromagnetic free layer sandwiched by first and second heterogeneous spacer layers, first and second ferromagnetic pinned layers proximate the first and second heterogeneous spacer layers respectively, and first and second AF layers adjacent to the first and second ferromagnetic pinned layers respectively.
Heterogeneous spacer layers can also be used in CPP multilayer magnetoresistive sensors according to another alternative embodiment of the present invention. A CPP multilayer magnetoresistive sensor can include multiple ferromagnetic free layers and multiple heterogeneous spacer layers, wherein each spacer layer is disposed between two ferromagnetic free layers. CPP multilayer magnetoresistive sensors produce higher resistances than simple CPP spin valves since each additional heterogeneous spacer layer will have a considerable resistance contribution to the total resistance of the CPP magnetoresistive sensors.
Several methods of making heterogeneous spacer layers are described in a second embodiment of the present invention. A well-known way to produce heterogeneous spacer layers is by co-depositing of an immiscible conductive material and an insulating or highly resistive material. In this method, the spacer layer can be produced from metals, such as Cu or Au, with oxides, such as SiO2 or Al2O3. The spacer layer can also be produced from co-deposited Au and Si.
Another method of making heterogeneous spacer layers involves a two steps process. A layer of a heterogeneous material of two metals, one of which resists oxidation, is first sputtered-deposited. This heterogeneous layer is then exposed to oxygen so that one metal is oxidized while the other metal remains metallic form.
An additional method of making heterogeneous spacer layers involves depositing a discontinuous insulating or highly resistive layer on top of a conductive layer. The holes in the discontinuous layer result in limited current paths for the electrons. A similar method involves depositing a discontinuous conductive layer on top of another conductive layer that resists oxidation. The structure is exposed to oxygen to oxidize the discontinuous layer and thus produce a highly resistive matrix.
A further method of making heterogeneous spacer layers involves depositing a homogeneous metal layer, such as Cu layer. This metal layer is then partially oxidized to form a heterogeneous layer including conductive grains of un-oxidized metal pinholes within a highly resistive matrix of metal oxide.
CPP magnetoresistive sensors with heterogeneous spacer layers of the types described in the first and second embodiments can be incorporated in CPP GMR read/write heads according to a third embodiment of the present invention. A CPP GMR read/write head includes a first shield contacting a first conductive gap, a second shield contacting a second conductive gap, and a CPP magnetoresistive sensor with heterogeneous spacer layer disposed between the first and the second conductive gaps.
CPP GMR read/write heads of the type as described in the third embodiment can be incorporated in disk drives according to a fourth embodiment of the present invention. A disk drive includes a magnetic recording disk, and a CPP GMR read/write head having a CPP magnetoresistive sensor with heterogeneous spacer layers. The disk drive further includes an actuator connected to the CPP GMR read/write head for moving the CPP GMR read/write head across the magnetic recording disk, and a mechanism for moving the magnetic recording disk relative to the CPP GMR read/write head.